PARENTAL INVESTMENT AND MATING SYSTEMS IN MAMMALS.

Abstract

Theory on mating system evolution (e.g., Darwin, 1874; Orians, 1969; Trivers, 1972; Williams, 1975; Borgia, 1979) suggests that females should choose mates which confer maximum fitness. If males contribute significantly to the care of the offspring, both male and female fitness may be maximized in monogamy. We expect monogamy to be more prevalent in mammals bearing altricial than precocial young since opportunities for male investment (sensu Trivers, 1972) are greater. High paternal care has been noted among many species of monogamous mammals, and several authors suggest that male investment in offspring is one of the principal advantages of a monogamous mating system (Clutton-Brock and Harvey, 1977, 1978; Crook, 1977; Kleiman, 1977; Ralls, 1977; Daly and Wilson, 1978; Thomas and Birney, 1979). Parental investment is defined by Trivers (1972) as any investment by the parent in an individual offspring that increases the offspring's chance of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring. The sum of the parental investment received by each offspring in a given time has been defined as the parental effort (Low, 1978). Together, parental effort and mating effort comprise reproductive effort (Low, 1978; Alexander and Borgia, 1979). In this paper we demonstrate that indeed mating systems in mammals are correlated with the degree of maternal investment in offspring at birth. Females of different species expend variable time and energy and incur different risks in producing offspring of varying states of maturation, thus the stage of development of neonates directly reflects maternal investment. The relative maturity of the neonate is commonly described by the terms altricial and precocial. Unfortunately, there is not the dichotomy that these terms imply and definitional problems have arisen as a result. Certainly, there is a considerable range of variation within both categories (Ewer, 1968). For example, it has been suggested that all species in the Bovidae bear precocial young (Ralls, 1977), whereas Estes (1974) claims that only those in the bovid tribe Alcelaphini bear precocial young. Because of this ambiguity, in addition to altricial and precocial classifications of the young, we examine two relative measures of maternal investment at birth: gestation period, and neonate weight divided by litter size, both standardized for body weight by regression analysis.